1. Field of the Invention
The present invention relates to a multimedia broadcast/multicast service (MBMS). More particularly, the present invention relates to a frequency selection method and a frequency selection apparatus, which can maintain the degree of congestion of cells and the load of signal transmission at appropriate levels when a MBMS service session starts in a frequency layer convergence (FLC) situation where the MBMS service is provided in a preferred frequency layer (PL).
2. Description of the Related Art
Nowadays, according to developments in communication technologies, a mobile communication system implementing a wideband code division multiple access (CDMA) scheme not only provides conventional voice services, but also provides packet service communications for transmission of mass data and multimedia broadcast/communications for transmission of multimedia services. Therefore, in order to support the multimedia broadcast/communication, a MBMS service is actively being pursued for enabling services from one or more multimedia data sources to a plurality of user equipments (UE).
The term “MBMS” generally refers to a service which transmits the same multimedia data to a plurality of receivers over a radio network. In such a MBMS service, radio transmission resources can be saved by sharing one radio channel with the plurality of receivers. The MBMS service supports multimedia transmission formats such as real-time images, voices, still images, characters and the like and can simultaneously transmit voice data and image data according to the transmission formats, which represents that the MBMS service requires mass transmission resources. In a case of the MBMS service, the same data must be transmitted to cells where the users are located, so a point-to-point (PTP) access or a point-to-multiple (PTM) access is established.
FIG. 1 is a diagram illustrating conventional network architecture of nodes which provide MBMS services in a mobile communication network. Here is illustrated an architectural example in which the MBMS services are applied to a 3rd Generation partnership (3GGP) Universal Mobile Telecommunication Service (UMTS) system, a standard 3rd asynchronous mobile communication scheme based on Global system for Mobile Communications (GSM) and General packet Radio Services (GPRS).
Referring to FIG. 1, UEs 161, 162, 163, 171 and 172 denotes terminal apparatuses or subscribers capable of receiving the MBMS services, and cell 1 (designated by reference numeral ‘160’) and cell 2 (designated by reference numeral ‘170’) represents physical or logical service areas controlled by node B, that is, a base station apparatus which wirelessly transmits MBMS related data to the subscribers. A radio network controller (RNC) 140 controls the cells 160 and 170, selectively transmits multimedia data to a specific cell, and controls radio channels set for providing the MBMS services. Accesses between the RNC 140 and the UEs 161, 162, 163, 171 and 172 are gained via a radio resource control (RRC) interface. The RNC 140, the Node B and the cells 160 and 170 are inclusively referred to as a UMTS Terrestrial Radio Access Network (UTRAN).
The RNC 140 is connected to a packet switched or packet service (PS) network, such as Internet, by a serving GPRS support node (SGSN) 130. Communications between the RNC 140 and the PS network implemented by packet switched signaling (PS signaling). In particular, an access between the RNC 140 and the SGSN 130 is referred to as an ‘Iu-PS interface’. The SGSN 130 controls MBMS related services of the respective subscribers. As a typical example, the SGSN 130 serves to manage data related to service charging for the subscribers and selectively transmit multimedia data to a specific RNC 140.
A transit network (transit NW) 120 provides a communication path between a broadcast/multicast service center (BM-SC) 110 and the SGSN 130, and may be connected to an external network through a gateway GPRS support node (GGSN) (not shown). The BM-SC 110, a source of MBMS data, is responsible for scheduling of the MBMS data.
The RNC 140 is connected to a circuit switched (CS) network by a mobile switching center (MSC) 150. The CS network signifies a legacy communication network which is centered on voice as an access basis. Communications between the RNC 140 and the MSC 150 is implemented by circuit switched signaling (CS signaling). In particular, an access between the RNC 140 and the MSC 150 is referred to as an ‘Iu-CS’ interface.
MBMS data streams generated by the BM-SC 110 reach the UEs 161, 162, 163, 171 and 172 through the transit NW 120, the SGSN 130, the RNC 140 and the node B/cells 160 and 170.
Although not shown in FIG. 1, a plurality of SGSNs may exist with respect to one MBMS service, and a plurality of RNCs may exist with respect to each of the SGSNs. Each of the SGSNs selectively transmits data to the plurality of RNCs, each of which, in turn, selectively transmits the data to the plurality of cells. To this end, a list of subordinate nodes to which data streams are to be transmitted (that is, a list of RNCs in a case of the SGSN and a list of cells in a case of the RNC) is stored such that the MBMS data are selectively transmitted later to only the stored nodes.
FIG. 2 is a diagram illustrating conventional procedures between a UE and a network, which are performed for a MBMS service. Here, a BM-SC 206 communicates with the UE 202 through a RNC 204. Also, the RNC 204 communicates with the BM-SC 206 through a SGSC (not shown). Although only one RNC 204 and only one UE 202 are shown in the drawing, it is obvious that the same procedures can be applied to a plurality of UEs desiring to receive the MBMS service and a plurality of RNCs controlling the plurality of UEs.
Referring to FIG. 2, a subscription step 210 represents a process in which a user desiring to receive the MBMS service, that is, the UE 202 is registered with a service provider, that is, the BM-SC 206. In the subscription step 210, the service provider (for example, the BM-SC 206) and the user (for example, the UE 202) exchange basic information related to service charging or service reception with each other.
In an announcement step 220, the UE 202 acquires basic information on the MBMS service. For example, the UE 202 detects MBMS IDs for discerning MBMS services, which the BM-SC 206 can provide to the UE 202, from each other, service initiation time, service duration and so forth. Here, the MBMS ID comprises a multicast address and an access point name (APN).
In the announcement step 220, nodes located between the BM-SC 206 and the UE 202, such as the RNC 204, the SGSN, a transit NW and the like, detect the UE 202 and nodes connected to the UE 202. For example, the SGSC detects a list of UEs desiring to receive the MBMS service at a lower level of the SGSN and a list of RNCs in which the UEs are located. Afterwards, the SGSN transmits MBMS data to the RNCs, in which the UEs are located, with reference to the lists.
The UE 202 having acquired the basic information on the MBMS service performs a joining step 230 for receiving the MBMS data. In the joining step 230, the UE 202 transfers at least one interest MBMS ID of the MBMS IDs acquired through the announcement step 220 to the BM-SC 206.
A notification step 240 represents a process for paging the UE 202 in order to notify the UE 202 that a session of the MBMS service, which the UE 202 joins, starts and the MBMS service will begin soon. In the notification step 240, group paging for a plurality of UEs joining the MBMS service is performed.
A radio resource allocation step 250 represents a process in which a radio resource is allocated between the UE 202 and the RNC 204 so as to actually provide the MBMS service, and information on the radio resource allocation is published to related nodes. In the notification step 250, the RNC 204 may determines a PTM transfer mode or a PTP transfer mode based on information on the number of UEs belonging to each subordinate cell and a radio resource management function.
In a data transfer step 260, the MBMS data are actually transmitted to the UE 202 through the RNC 204. When a need to change information related to the MBMS service exists, for example, a ciphering key in the data transfer step 260, the RNC 204 carries new information related to the MBMS service on MBMS control information to transmit the new information to all the UEs which are receiving the MBMS service.
If the MBMS session ends, in a radio resource release step 270, the allocated radio resource is released and the UE 202 is notified of the MBMS radio resource release. Although not shown in the drawing, the UE 202 may also spontaneously request to stop the reception of the MBMS service and stop the MBMS service reception on the way of receiving the MBMS service in step 250.
Recent MBMS systems support a FLC technology in which preferred frequency bands are allocated to each affordable MBMS services to enable UEs using the MBMS services to receive desired MBMS services at preferred frequencies. FLC information indicates the preferred frequencies corresponding to the respective MBMS services. At this time, a cell of a preferred frequency selected for a specific MBMS service is referred to as a preferred frequency layer (PL), and cells of other frequencies are referred to as non preferred frequency layers (NPL). Usually, only a corresponding MBMS service can be provided in the PL. However, it is not always possible to provide the corresponding MBMS service by the NPL. That is, in spite of the FLC, the corresponding MBMS service may be provided for UEs left to the NPLs.
In the FPL technology of the conventional MBMS system which operates as stated above, when the MBMS service starts, the UTRAN determines the PL of the MBMS service and transmits a message informing UEs located in subordinate cells of the MBMS service starting and the determined PL. In such a situation, since connected mode UEs, which subscribe to the MBMS service and are located in the NPLs of the MBMS service, move to the PL and then almost simultaneously attempt access for uplink signaling, there may be a problem in that uplink signaling congestion occurs.